The present invention relates to a fluid reservoir for a hydraulic brake master cylinder, and more particularly to an improvement of a sealed fluid reservoir in which brake fluid is constantly sealed against the surrounding air regardless of increase or decrease of the brake fluid.
In conventional sealed reservoirs of this kind, an extendable diaphragm seal member is fluid-tightly assembled with the opening of the reservoir casing to seal brake fluid stored within the casing against the surrounding air and to prevent entrance of water into the brake fluid. In adaptation of the diaphragm seal member, the fluid reservoir becomes large in size as the interior of the casing is occupied by the diaphragm seal member. Furthermore, in case the diaphragm seal member is designed in a thin film form to minimize the volume of the seal member so as to form the reservior in a possible smaller size, the diaphragm seal member becomes less durable and high in production cost due to difficulty of fabrication of the seal member.